Aluminum is subject to a phenomenon known as crevice corrosion. This phenomena is well known and occurs in crevices at joints of fluid handling devices made of aluminum. Typically, crevice corrosion manifests itself by the appearance of pitting of the aluminum metal at a crevice where aluminum metal is joined or joins another material in gasket sealed arrangement. Such pitting can eventually lead to the formation of small holes in the aluminum and to a leakage failure at the crevice.
In the manufacture of fluid handling devices such as automotive heat exchangers and the like, it is common practice to assemble a first aluminum component such as a heat exchanger header sheet together with a second component manufactured from the same or another material, such as a polymeric header tank or the like. For manufacturing convenience, assembly can involve locking the second component in fluid tight, gasket sealed relationship with the first aluminum component by crimping or otherwise forming the first aluminum component about an edge or surface of the second component to deflect a sealing gasket between the components and thus form a fluid seal therebetween. Such assembly typically results in the formation of an elongated crevice between surfaces of the two components which when formed within the fluid handling compartment can be a site of corrosion failure of an aluminum component.
It has been found that with aluminum materials, when a fluid containing dissolved oxygen and an electrolyte stagnates in a crevice, the crevice can corrode by what is believed to be an electrolytic activity. In typical automotive heat exchanger applications, it is not unusual for automotive water-antifreeze solutions to contain enough dissolved oxygen and impurities to foster an electrolytic activity and promote crevice corrosion.
It is well known that dissolved oxygen will typically react with aluminum components to form an oxide on an exposed aluminum wall. Generally, it is anticipated that such reaction will produce a barrier coating which will act in a self limiting manner to resist corrosion failure. One recurring problem however is that of joint crevice corrosion. It has been found that material thicknesses adequate to avoid general failure of the component from anticipated mechanical load and corrosion due to the bulk fluid may be inadequate to protect the component from crevice corrosion.
We have found that crevice corrosion can be affected by the configuration of the crevice in an automotive cooling system. When the crevice is configured to allow a water-antifreeze solution to be held stagnant within the crevice, corrosion appears to occur much more rapidly than if the solution within the crevice substantially intermixes with the bulk of the fluid being handled by the system. Though we do not wish to be bound by the following, it is believed that when a water/antifreeze solution is isolated in a crevice such that it does not substantially mix with the bulk of the mixture being circulated through the assembly that crevice corrosion is electrolytically fostered. It is believed that this is at least in part due to an inhibited replacement of depleted dissolved oxygen in stagnant, isolated fluid in the crevice. By that is meant that as the dissolved oxygen in stagnant, isolated fluid of a crevice reacts with the aluminum to form an oxide or reacts with other reactants, it is not rapidly replenished with dissolved oxygen from the circulating bulk mixture because of its stagnant isolation. Thus, the stagnant fluid within the crevice is oxygen deficient as compared to the fluid within the bulk circulating mixture.
It is known that oxygen deficient water is electro-positive in relation to water containing dissolved oxygen. It is also known that in an electrolytic environment, electrons will flow to an electro-positive portion of the environment tending toward charge conservation of the overall environment. Thus, in a bulk mixture wherein oxygen in a fluid medium is reacting with aluminum or other reactants, oxygen from throughout the bulk mixture would continually replenish oxygen at the reaction interface thus tending to stabilize the mixture. It is believed however, that within the electro-positive environment of the oxygen depleted mixture of stagnant, isolated fluid in the crevice, the stagnation of the fluid in the crevice inhibits dissolved oxygen replenishment from the bulk mixture. This inhibition to oxygen replinishment interrupts the normal charge conservation mechanism, and electron flow from the aluminum to the water within the crevice becomes the compensating charge conservation mechanism. Such compensating flow of electrons from the wall to the fluid, can act to remove aluminum from a wall of the crevice resulting in the formation of pits. The pits may eventually penetrate the wall of the crevice and constitute sufficient crevice corrosion to cause failure of the assembly.
An object of the invention is to provide a method for reducing crevice corrosion in aluminum devices.
Another object of the invention is to provide an automotive heat exchanger having corrosion inhibited crevices therein.
These and other objects of the invention will become apparent from the following description.